Heavy crude oils typically contain asphaltenes. Asphaltenes are organic heterocyclic macro-molecules that usually represent the heaviest compounds in crude oil. Asphaltenes are defined as a solubility class and, whilst they are typically soluble in aromatic solvents such as toluene, they are insoluble in paraffinic solvents such as n-heptane.
Under normal conditions (i.e. atmospheric temperature and pressure), the asphaltenes in a crude oil are generally stable and remain in solution. However, during refining, crude oil is typically passed across a number of heat exchangers before being fed to the main Crude Distillation Unit (CDU). Generally, the crude oil will be subjected to a ‘cold pre-heat’ which involves passing the crude oil across a series of heat exchangers to bring it to an initial temperature of 90-140° C., followed by a desalting stage. The oil is then further heated in a bank of heat exchangers to a temperature of 230-270° C. This stage is known as the ‘hot pre-heat’. The crude oil is then passed to a furnace for further heating before it is passed to the CDU.
During the entire cold and hot pre-heating stages, the crude oil undergoes thermal expansion. The solvent power of the crude oil is inversely related to the molar volume of the crude oil. Thus, as the crude oil undergoes thermal expansion, the molar volume increases and the solvent power drops. If the solvent power falls below the critical solvent power of the crude oil, asphaltenes in the crude oil become unstable and precipitation may be observed. As the precipitate begins to adhere to metal surfaces in the refinery, fouling occurs.
Furthermore, there is a gradual pressure drop throughout the pre-heating stages, and crude oil which started at a pressure of 30-60 bar is typically only at a pressure of 15-30 bar at the end of the pre-heating stages. A drop in pressure lowers the effective boiling point of the crude oil, exacerbating the effects of the thermal expansion.
Crude oil is typically closest to its boiling point in the heat exchangers located immediately upstream of the furnace, and so the solvent power of the crude oil is at its lowest at this stage of the pre-heating process. Fouling rates therefore tend to be the highest in these heat exchangers.
Significant fouling is also observed in the furnace. However, once the crude oil has entered the furnace, components of the crude oil begin to vapourise. As components are evolved from the oil, the solvent power of the remaining liquid phase increases and asphaltene precipitation risks are generally lowered.
One of the biggest challenges faced by refineries that process crude oils is the ability to ensure that the asphaltenes in the crude oil are kept stable and in solution throughout the refinery.
Typical methods for assessing fouling risk of crude oils or blends thereof involve carrying out laboratory tests at atmospheric pressure and at temperatures of up to 60° C. These ambient condition lab measurements are used to generate compatibility parameters (i.e. solvent power and critical solvent power) from which the risk of asphaltene precipitation during crude oil processing may be predicted.
One method for reducing the risk of asphaltene precipitation is to ensure that the solvent power of a crude oil or a blend of crude oils is 5-15% higher than the critical solvent power (see US 2004/0121472).
However, high levels of fouling have been observed in crude oil blends which were believed to have an adequate margin between the solvent power and the critical solvent power, including crude oil blends with a ratio of solvent power to critical solvent power of greater than 1.30. Accordingly, a fixed margin between solvent power and critical solvent power is not, in some cases, enough to prevent unwanted asphaltene precipitation. Moreover, the margin between the solvent power and the critical solvent of a crude oil blend can have implications for the economics of a refinery, as too high a margin can constrain the feedstocks that may be employed in the refinery. Whilst a suitable margin between solvent power and critical solvent power can be determined by trial and error, this can be costly in terms of time and equipment.
Accordingly, there is a need for an improved method for predicting the ratio of solvent power to critical solvent power at which asphaltene precipitation occurs, so as to avoid unexpected fouling in a refinery.